1. Field of the Invention
The present invention relates to a constant current source and a solid imaging apparatus using the same.
2. Description of the Related Art
In recent years, as a solid imaging apparatus, a CMOS type solid imaging apparatus (hereafter, referred to as a CMOS sensor) has widely been used. According to the CMOS sensor, photocharges generated in a photoelectric conversion unit are transferred to a floating diffusion one row by one and signals are simultaneously read out of vertical read-out lines to a signal processing unit on a row unit basis by using a source-follower of each column. A constant current source of each column for driving each source-follower is generally biased by a common current source circuit. If current noise components having a correlation exist in an output current of each constant current source, output signal noises common to each column are generated and are recognized as horizontal stripe noises in an image. Therefore, it is necessary that the constant current source which is used in the CMOS sensor is a low-noise circuit, and it is necessary to reduce the noises which are generated in the common current source circuit.
The Japanese Patent Application Laid-Open Gazette No. 2007-129473 discloses such a technique that in order to reduce an electric potential fluctuation of a vertical read-out line which is caused by exogenous noises, a resistor (7 in FIG. 1) is connected to a common gate line (5 in FIG. 1) of MOS transistors of the constant current source. According to such a technique, the noises which are generated in the common current source circuit (4 in FIG. 1) are also simultaneously reduced.
A current value of the common current source circuit disclosed in the Japanese Patent Application Laid-Open Gazette No. 2007-129473 fluctuates in accordance with a resistance value, a variation in characteristics of the transistors, and a power source voltage to be used. Generally, in the CMOS sensor manufactured by a semiconductor process, there is a variation of tens of % with respect to the resistance value and there is a variation in a range from tens of mV to about 100 mV with respect to a threshold value of the transistor. In the use of CMOS sensor, since there is also a case where a different power source voltage is set for every product, a variation in current value of the constant current source is large. As the current of the constant current source increases, an increase in current consumption is caused. On the contrary, as the current decreases, a driving power of the source-follower is reduced and a read-out speed of the pixel is decreased.